Acoustic construction



Feb. ll, 1936.

V. A. SCHLENKER ACOUSTIC CONSTRUCTION Fi1ed Feb. 2, 1951 .2 sheets-sheet 1 Feb. l1, 1936.

V. A, SCHLENKER Y ACOUSTIC CONSTRUCTION Filed Fb. 2, 1931 2 sheets-sheet 2 /N 15E/v TOR %2A TTORNEY Patented Feb. 1l, 1936 UNITED STATES PATENT OFFICE one-half to Edward N. Y.

H. Cumpston, Brighton,

Application February 2, 1931, Serial No. 512,921

15 Claims.

This invention relates to an acoustic construction and has for one of its objects the provision of a construction which may be used to control sound Waves in a predetermined desired manner.

Another object of the invention is the provision of a construction which will selectively absorb 4sound waves of different Wave lengths and especially of one which will absorb a greater proportion of waves longer than a predetermined Wave length than of Waves shorter than the predetermined length.

Still another object is the provision of a construction which is simple, easy to construct and install, comparatively inexpensive, substantially reproof, and generally satisfactory.

A further object is the provision of a construction of pleasing appearance, so that theatres, auditoriums, churches, or other buildings may employ the construction of the present invention without detriment to their appearance.

A still further object is the provision of a simple and satisfactory method of controlling acoustic conditions.

To these and other ends the invention resides in certain improvements and combinations of parts, all as will be hereinafter more fully described, the novel features being pointed out in the claims at the end of the specification.

In the drawings:

Fig. l is a fragmentary vertical section taken transversely through an acoustic screen constructed in accordance with one embodiment of the invention;

Fig. 2 is asimilar view illustrating a fragment of the Wall constructed in accordance with another embodiment of the invention;

Fig. 3 is la view similar to Fig. 2 showing still another embodiment of the invention;

Fig. 4 is an elevation lor face view of either of the embodiments shown in Figs. l and 2;

Fig. 5 is a view similar to Fig. 2 illustrating another embodiment;

Fig. 6 is a similar View illustrating a slightly different form of construction;

Fig. 7 is a similar view illustrating still another embodiment of the invention, and

Fig. 8 is an elevation or face view of the construction shown in Fig. 7.

Similar reference numerals throughout the several views indicate the same parts.

Heretofore many attempts have been made to control acoustic conditions and in some instances such attempts have been satisfactory under certain predetermined conditions or in certain surroundings. In many other instances, Y'id nvaver,

has been exceedingly diflicult if not impossible to obtain, by the prior constructions, uniform absorption of all wave lengths, and so far as I v am aware no prior construction suitable for practical use in buildings has been effective in aA practical way to absorb greater proportions of sounds of low frequency than of sounds of higher frequency.

My acoustical investigations and experiments indicate that in many auditoriurns, theatres, and the like, there is no need for treatment of high frequency sounds, but that there is great need, in order to improve acoustic conditions, for treatment of sound waves of relatively low frequency. Furthermore, so far as articulation is concerned, it is generally advantageous to absorb greater proportions of sound waves of loW frequency than those of high frequency, in order to diminish the loudness of the low tones which contribute very little to the articulation and which tend to mask and obscure the high frequency components of speech. In practice I find that articulation is actually improved by absorbing more energy from the low frequency sound waves than from the high frequency ones.

There is no sharp dividing line between the low frequency sounds and the high frequency sounds as these terms are used in this specication and in the claims. The predetermined frequency which is selected as the one below which it is desired to absorb a greater proportion of the sound Waves may be varied as desired. In practice, it is found to be satisfactory to place this predetermined frequency at about seven hundred one thousand cycles per second, and in practice also the change from the sounds of which a greater part is absorbed to the sounds of which a. lesser part is absorbed is not abrupt, but is gradual over a range of perhaps an octave or half an octave. Thus it is ordinarily found to be Satisfactory to absorb a considerable proportion of sound waves below a frequency of about six or seven hundred cycles, and to absorb only a comparatively slight proportion of the sound waves above about a thousand cycles per second.

The present application discloses a. number of possible embodiments of the invention which may be used satisfactorily to produce this desired greater absorption of low frequency sounds than of high frequency sounds. All of these embodiments are based more or less on the same general principle of the use of sound absorbing material of which a portion is acoustically masked off so that sound waves will not fall upon this portion of the material, while another portion of the absorbing material is unmasked so that sound waves may impinge thereon.

The relative sizes and arrangement of the masked and unmasked areas bear a definite predetermined relation to each other depending upon the frequency or wave length of the sounds which it is desired to absorb. Thus, according to the preferred method, the sound absorbing material has a masking coating applied thereto, which coating may be described as apertured or discontinuous, and the sizes and arrangements of the apertures or the boundaries of the coating are not left yto chance or whim, but are carefully chosen with regard to the acoustic properties desired.

It is found in practice that sound waves impinging upon a surface having apertures therein will flow through the apertures to an extent depending somewhat upon the wave length of the sound. I further find in practice that sound waves having a wave length equal to or greater than twice the distance between apertures will flow through the apertures more readily than sound Waves having a wave length of less than twice the distance between apertures. By applying this principle, according to the present invention, to an acoustic structure, the structure can be made to absorb a greater proportion off sound waves longer than a predetermined wave length than of sound Waves shorter than this predetermined wave length., and the predetermined wave length will be approximately twice the distance between the apertures.

It is also found that where sound waves impinge upon a surface having an aperture, the energy flowing through the aperture is considerably more than the energy of that part of the wave front which has the same area as the aperture. In other words, energy from a substantial area surrounding the aperture appears to be drawn toward the aperture and passes therethrough, somewhat as though the aperture were in the nature of a funnel. This area from which energy is drawn appears to be more or less in.- dependent of the size of the aperture (assuming that the aperture is of reasonable size to permit the passage of substantial energy therethrough) but does depend upon the wave length of the sound. It can be demonstrated both mathematically and by experiment that the area from which energy is drawn to the aperture is approximately equal to the square of the wave length of the sound divided by pi. In other words, the area from which energy is abstracted by the aperture may be expressed by the formula in which w is the wave length of the sound and d2 is the area, or d is the square root of the area or the length of one side of a square having the area from which the energy is abstracted.

Thus, if the surface be provided with a series of regularly spaced apertures, the energy of sound waves impinging thereon will flow to a relatively large extent through the apertures Whenever the sound waves have a wave length the square of which divided by pi is approximately equal to or greater than the square of the distance between the apertures, whereas relatively smaller amounts of energy will flow through the apertures when the wave lengths are less than this.

By applying these principles, a selective sound absorbing construction may be made which will absorb greater proportions of 'sound waves of low frequency or long wave length than of sound waves of high frequency or short wave length. The results obtained by applying this principle just set forth are approximately the same as those obtained by the principle previously set forth with regard to the distance between apertures being half thewave length of the sound, and thus approximately the same characteristics may be obtained by following either of these principles or by using both together.

For example, selecting the frequency of a thousand cycles as the predetermined frequency below which it is desired to absorb a greater proportion of sound waves than above this frequency, the wave length corresponding to a frequencyof one thousand is about 1.12 feet. Applying the formula the distance between apertures then becomes 0.56 feet. Applying the other formula that gives a value to d of 1.12 feet, or between 13 and J 14 inches. Applying the formula (1% 1.28 feet or between l5 and 16 inches. Hence, if the apertures are spaced about 14 or 15 inches apart in both directions, theoretically substantially all the energy of sound waves below five hundred vibrations per second would flow through these apertures, while sound waves above ve hundred vibrations per second would ow through to a considerably less extent.

In any construction of this general type having apertures in a surface, if sound absorbing material is placed behind the apertures, the sound energy flowing through the apertures Will be absorbed thereby to a greater or lesser extent depending on the efficiency of the absorbing material, and reflection of the sound waves back through the apertures will thus Abe reduced. According to the present invention, such sound absorbing material is provided behind the apertures, the character and efficiency of the material being chosen in accordance with the results desired. For example, if it is desired to absorb as much of the low frequency sound energy as possible, a sound absorbing material of the greatest eiliciency practically obtainable may be employed behind the apertures. On the yother hand, if it is desired to absorb only moderate amounts of the sound energy flowing through the apertures, an absorbing material of lower efliciency will be employed. Consequently the term sound absorbing material and similar expressions used in this specification and in the claims have a purely relative meaning, rather than an absolute one, and it is intended that expressions of this kind shall include any material performing the desired functions whether it be of high or low eiciency in absorbing sounds.

'I'he surface between the apertures may be characterized in general as a sound reflecting surface, in that it is intended to reflect more or less of the sound energy falling thereon, as distinguished from the energy flow-ing through the apertures, which is more or less absorbed. Here again, as in the case of the sound absorbing material, the efficiency of the sound reflecting material may be varied greatly in order to achieve the desired results. For example, if it is desired to reflect as much as possible of the energy of the high frequency sound waves the surface between the apertures will be made of as high reflecting efficiency as possible. On the other hand, if it be desired to deaden partially the high frequency waves, the eiciency of the reflecting surface will be made lower, as by covering it with a layer of porous material or fabric, or by other` known methods.

Consequently the term sound reflecting material and similar expressions `as used in this specification and in the claims, are intended to cover any material performing the general purposes and functions herein described, whether such material be of high or low efficiency in reflecting sound waves.

In connection with the discussion given above regarding ow of energy through an aperture, it was stated that the size of the aperture theoretically made no diiference in the area from which energy was drawn, provided the aperture were sufliciently large to permit the energy to flow through without undue resistance. The size of the aperture does have some importance in a practical acoustical construction, however, because it determines to some extent the proportions of sound waves which will be reflected and absorbed.

It can readily be seen that the larger the area of the apertures, the greater will be the proportion of sound waves which fall directly on the apertures and are absorbed by the absorbing material behind, and the less will be the area of the sound reflecting surface capable of reflecting the waves falling thereon. In actual use, satisfactory results have been obtained from a construction having circular apertures 3 inches in diameter spaced at intervals of 10 inches in each direction. Thus the area of the surface applicable to each aperture is 100 square inches while the area of the 3 inch diameter aperture itself is 7.1 square inches, the apertures thus roughly occupying about 7% of the total surface. Hence,

. about '7% of all sound waves, irrespective of their frequency, will fall upon the area of the apertures and be absorbed, and about 93% of the sound Waves would fall upon the reflecting surlface and would be reflected were it-not for the flow of energy through the apertures in accordance with the wave lengths of the sound, as above set forth. If the apertures were made larger than this, they would occupy an increased proportion of the total area, and thus an increased proportion ofall sound waves of all frequencies would flow through the aperture and be absorbed. Thus by varying the sizes of the apertures, different characteristics can be obtained to some extent in accordance with the results desired. In general, however, it will be found most satisfactory to use apertures of about the sizes and proportions above mentioned, and preferably occupying not over 16% or 15% of the total area. The apertures may be of any shape desired, and are not necessarily circular.

Referring now to Fig. 1 of the drawings, there is illustrated a construction which has been found to be satisfactory in practice and which is made up in the form of what might be termed an acoustic screen. Secured to any suitable framework are two plates Ill and I I substantially parallel and spaced from each other. They may be of any suitable and preferably relatively light material, such as a veneer plyboard, wallboard, or the like. The layer II has apertures I2 therein, while the space between the layers I0 and II is filled with sound absorbing material I3 such as mineral wool, for example. Preferably also a covering layer is employed over the surface I I to give it `a uniform appearance and hide the apertures therein. This covering layer in the present embodiment is in the form of a layer I4 of textile fabric such as damask, although obviously other materials could be used. Such a textile layer does not seriously interfere with the flow of sound energy through the apertures I2 o-r with the reflection of sound waves from the unapertured portions of the surface I i, yet it adds materially to the appearance of the construction. The covering layer I4 can be decorated or colored in any desired manne-r.

It is found in practice that a satisfactory construction for many purposes is provided when the holes I2 are three inches in diameter and are spaced ten inches apart center to center in` both directions, as shown in Fig. 4.

The invention contemplates that acoustical screens of this kind may be made up as small portable units of any desired size which may be carried or otherwise shifted from place to place and set up in` any desired position either vertically or otherwise, depending on` the results to be obtained. Such acoustic screens would be useful, for example, in moving picture studiosfor the taking of talking pictures, the screens being rapidly shiftable in position and arranged in any desired combination until exactly the right acoustical condition of the studios is obtained.

Furthermore, such screens can be made substantially reproof. Where the filling I3 is of mineral wool, which is substantially noncoinbustible, the fire hazard is practically negligible: because it would be' difficult to set the layers IE) and II on re, and if absolute reproefness is necessary under exceptional circumstances, the layers I0 and I I could be made of plywood or wallboard which had been treated to render it iireproof, or even of thin metallic sheets ii necessary. Furthermore, the covering 'layer i4 can be omitted whenever the appearance of the screen is not material, thus doing away with any combustible material which might otherwise be used for the layer I4.

Actual tests made with a screen constructed of plywood layers I and and a lling I3 of rock wool approximately 11/2 inches thick, with the apertures of the size and spacing above mentioned, have shown that the absorption of sound energy by such a screen is extremely selective and depends upon the Wave length of the sound. The tests indicate that for frequencies of over one thousand cycles, less than fifteen percent. of the sound energy is absorbed, whereas for frequencies of under five hundred cycles, from about forty-five to sixty-five per cent. of the energy is absorbed, thus confirming the theoretical principles which have been set forth above.

The same construction, instead of being made up in the form of a portable acoustic screen, can be applied permanently to a building, either by fixing the entire screen as shown in Fig. 1 to a wall or ceiling, or by omitting the layer I0 and placing the filling I3 directly in contact With the wall or ceiling, which co-nstruction is ordinarily .preferred Fig. 2 illustrates the application to a Wall of a slightly modified construction, although this or any of the other embodiments illustrated could also be made in the form of a. portable acoustic screen if desired. In Fig. 2, the wall or ceiling is represented at 20, the layer of reflecting material at 2|, the apertures therein at 22, and the filling of sound absorbing material at 23. This absorbing material may be rock wool, as before, or any other desired material.

. In this form of construction, the layer 2| is made of a sheet of paper, such as heavy kraft paper or the like. The layer of paper acts as a reiiecting surface for reflecting the high frequency sound waves, while the energy of the lower frequency waves fiows through the openings 22 and is absorbed by the filling 23.

If it is desired to cover the reflecting layer in some way, so that the apertures are not visible, this may be done in the same manner shown in i Fig. 1, or by other suitable means. In the embodiment shown in Fig. 2, there is illustrated a layer 24 of metal lath placed in front of the paper 2|, to which lath is applied a layer 25 of porous acoustic plaster, made for example, by mixing rock wool with a suitable binder such as clay. This plaster 25 is so thin and of such a porous nature that it does not interfere to any great extent with the passage of sound waves through it to the paper 2| and the apertures therein, but it serves to mask the paper 2| and the apertures from observation. Suitable decoration may be applied to the exposed surface of the plaster.

In a construction of this kind, when it is desired to absorb substantial quantities of sound energy from sounds of all frequencies in addition to the selective absorption of greater energy from sound waves of lower frequencies, then the acoustic plaster 25 may be varied in composition or made thicker so that it will absorb substantial quantities of all sound Waves passing therethrough. In practice it is found that a layer of acoustic plaster 11g inch thick to 1A, inch thick does not seriously interfere with the passage of the sound waves therethrough. When it is desired to absorb substantial quantities of the sound energy of all frequencies, the layer 25 can be made substantially thicker than 1/8 inch or made of more absorptive material.

Fig. 3 illustrates still another possible embodiment of the invention in which the wall or ceiling is represented at 30 and the filling of sound absorbing material at 33. Here the metal lath 34 is placed directly against the sound absorbing material 33 without the interposition of the reecting layer, and a layer 35 of acoustic plaster is applied to the lath 34. This layer 35 may be relatively thin, say g inch to 1A; inch thick, its purpose being to provide a smooth surface to which paint may be applied. The apertured reflecting layer in this embodiment of the invention comprises the layer 3| of thick non-porous paint applied to the plaster 35 in a discontinuous manner; that is, with apertures or unpainted spaces arranged in any desired manner' according to the results desired, such as the manner in which the apertures I2 and 22 are arranged in the embodiments previously described.

This layer of heavy non-porous paint 3| performs substantially the same function as the layers and 2| of the embodiments previously described. That is, the high frequency sound waves impinging upon the paint layer 3| Will not flow through the layer to a material extent because of the non-porous nature thereof, but will be reflected therefrom. Low frequency sound waves, on the other hand, will have a substantial portion of their energy flow through the unpainted spaces o-r apertures of the layer 3| and through the thin acoustic plaster 35 and into the filling 33 in which they will be absorbed. If the acoustic plaster 35 were made thicker so as to have the requisite absorbing capacity, the filling 33 might be omitted or reduced in thickness. If it is desired to mask the exposed surface so that the unpainted spaces will not be readily detected visually, this may be done by applying a decorative layer in the form of porous paint 36 which will not substantially interfere with the sound Waves, this layer 36 being painted over the paint layer 3| and over the plaster 35 in the apertures of the layer 3| to provide a uniform finish coat, or a coat decorated in any desired manner, so that the apertures in the layer 3| cannot be seen. Obviously the finish layer 36 of porous paint may be applied in any design or pattern desired. In Fig. 3, the layers of paint are shown greatly exaggerated in thickness for the sake of clearness.

Still another embodiment of the invention is illustrated in Fig. 5. Here the wall or ceiling is represented at 50 and the reflecting layer at 5|, which may be of any desired form, such as a layer of paper similar to the layer 2| illustrated in Fig. 2, and having apertures 52 therein. As in Fig. 2, metal lath 54 may be applied over the paper and a thin layer of acoustic plaster 55 may be spread on the lath 54. The diierence between this construction and that shown in Fig. 2 is that the filling 53, instead of extending throughout the entire area behind the layer 5|, is concentrated or bunched behind the apertures 52 and is omitted from the spaces between the apertures, resulting in considerable saving of material Without any great loss of efficiency. This same principle of applying the filling only behind the apertures, or to any desired part only of the entire area, may be employed in connection with any of the other forms of the invention.

In Fig. 6 is illustrated another form of the invention in which the Wall or ceiling is shown at 6|), the reflecting layer at 6|, the apertures at 62, and the lling at 63. In this embodiment, the reflecting layer is covered and masked by a layer 64 which may be, for example, a thin blanket of rock wool. Such a rock wool blanket of from 11g inch to 1/8 inch thick will give satisfactory results under some conditions, the thickness being varied as desired, depending upon the results to be obtained. For example, as the thickness of the blanket 64 is increased, greater proportions of all sound waves will be absorbed, so that the selective absorption of low frequency sound waves will be less and less apparent.

In any of these constructions, various proportions of the various layers and materials may be used to lobtain whatever results are desired under the particular circumstances. For instance, wherever it is desired to render the absorption as selective as possible, reflecting as much as possible of the high frequency sounds and absorbing as much as possible of the low frequency sounds, the outer layer such as I2, 25, 36, 55, or 64 will be reduced to a minimum thickness and sound absorbing capacity, or entirely omitted. When it is desired to make the selective effect less pronounced and to absorb considerable quantities of all sounds and only slightly more of the low frequency sounds than of the high frequency sounds, then the layers such as I2, 25, 36, 55, .or GII in front of the reu fleeting surface will be made thicker or of greater sound absorbing capacity so as to absorb substantial amounts of all sounds in addition to the selective absorption of low frequency sounds obtained by the use of the apertured reecting surface.

The sound absorbing capacity of the outer or covering layers can, if desired, be carried to a point Where there will be practically uniform absorption of sound of all frequencies and little or no selective absorption. This is advantageous under some circumstances, and may be accomplished by the use of the present invention, as above mentioned, whereas the use of sound absorbing material alone, without the apertured reflecting surface of the present invention, frequently gives a selective absorption in which the high frequency sounds are more absorbed than the low frequency ones.

Again, the results achieved may be varied by varying the thickness, density, or sound absorbing capacity of the lling layers such as I3, 23, 33, etc. In brief, practically all of the dimensions and proportions are variable as desired tol pro-duce exactly the results desired, while yet falling Within the broad principles of this invention.

Figs. 7 and 8 illustrate still another form of the invention. It is frequently desired to decorate walls or ceilings of auditor1ums, churches, theatres, or the like in a manner to resemble stonework. In the embodiment shown in Figs. '1 and 8, the wall or ceiling is illustrated at 1. Porous sound absorbing blocks 13 are applied to the wall or ceiling surface and aflixed thereto by any suitable means, such as the usual mortar or cement 14. These blocks 13 may be either natural or artificial stone, brick, tile, etc., of a porous nature so that they are more o-r less sound absorbing. The edges of the individual blocks are beveled, as indicated at 15, and the faces of the blocks, except for these beveled portions, are coated as illustrated at 1I with thick nonporous paint or other suitable material, which will tend to reflect sound waves rather than to permit them to pass through the coating into the blocks 13.

It will be seen that this form of the invention employs the same principle of a discontinuous re.- flecting surface having apertures therein., the apertures in this instance being constituted by the beveled spaces 15 to which the layer 1I is not applied. Thus the high frequency sound Waves will be reflected to a substantial extent from the coating 1I, While energy of the low frequency Waves will flow to a substantial extent through the apertures and into the beveled edges 15 of the porous blocks, in which it will be absorbed. The predetermined frequency below which more absorption will take place than above, will depend on the shape and size of the reflecting areas 1I with relation to the absorbing areas 15. The eiciency of the absorption of the low frequency sounds will also depend, to a considerable extent, on the porosity of they blocks 13, If the material of which these blocks are made is exceedingly porous, the sound will readily enter through the beveled edges 15 and be absorbed therein, Whereas when the blocks are less porous, less sound will enter, and a greater amount of the sound energy will be reflected from the edges 15.

It is obviously not necessary that the uncoated edges be beve-led, but they may be of any shape or form desired, and the uncoated portions need not necessarily be at the edges of the blocks, but can be placed at any desired points thereon.

In all of the various embodiments of the invention described, the layers such as I I, 2|, 3|, 5I, 6I, and 1I might be described as layers acoustically masking part of the sound absorbing material beneath, while allowing sound waves to impinge upon other portions of the absorbing material. Furthermore, the layers such as I4, 25, 36, 55, and 64, might b-e described as layers visually masking the acoustic masking layer and the apertures therein.

The term wall as used in this specification and in the accompanying claims is intended in a broad sense as including any wall structure irrespective of its position or orientation, whether horizontal, vertical, or inclined, and specifically as including top and bottom Wall structures (commonly known as ceilings and floors) as well as side wall structures.

While certain embodiments of the invention have been disclosed, it is to be understood that the inventive idea may be carried out in a nurnber of ways. This application is therefore not to be limited to the precise details described, but is intended to cover all variatie-ns and modifications thereof falling Within the spirit of the invention or the scope of the appended claims.

I claim:

l. A selective sound-absorbingl construction for absorbing a greater proportion of sound waves longer than a. selected Wave-length than of sound waves shorter than said selected wave-length, said construction comprising means forming a wall having areas of relatively high sound-absorbiing capacity with spaces between them approximately equal to half of said selected Wave-length.

2. A selective sound-absorbing construction for absorbing a greater proportion of sound waves longer than a chosen wave-length than of sound Waves shorter than said chosen wave-length, said construction comprising means forming a wall having porous areas for absorbing a relatively large proportion of sound waves falling upon them and substantially non-porous areas of substantial size between said porous areas, each porous area serving a non-porous area having an order of magnitude approximating the square of said chosen wave-length divided by pi.

3. A selective sound absorbing construction for absorbing a greater proportion of sound waves longer than a selected wave-length than of sound waves shorter than said selected wavelength said construction comprising means forming a wall having areas of greater soundabsorbing capacity and areas of lesser soundabsorbing capacity, the areas of lesser soundabsorbing capacity being of an order of magniture approximating the square of said selected wave-length divided by pi.

4. A selective sound-absorbing construction for absorbing a greater proportion of sound waves longer than a chosen wave-length than of sound waves shorter than said chosen wave-length, said construction comprising means forming a Wall having interspersed sound absorbing areas and sound reflecting areas, each of said sound reecting areas having an Larea approximately the square of said chosen wave-length divided by pi.

5. A selective sound-absorbing construction for absorbing a greater proportion of sound waves longer than a selected wave-length than of Sound waves shorter than said selected wave-length, said construction comprising a layer of material of lesser sound-absorbing capacity having apertures therein, and material of greater sound-absorbing capacity behind said apertures in position to absorb sound waves passing through said apertures, the distances between said apertures being of a magnitude approximating one-half of said selected Wave-length.

6. A selective sound-absorbing construction for absorbing a greater proportion of sound waves longer than a selected wave-length than of sound waves shorter than said selected wave-length, said construction comprising a material of relatively great sound-absorbing capacity, and a material of relatively less sound-absorbing capacity covering and masking a part of said material of great sound-absorbing capacity, said material of less capacity having a plurality of apertures of substantial size therethrough so that sound waves may pass through said material of less capacity to said material of great capacity, the area of said material of less capacity associated with each of said apertures being of a magnitude approximating the square ci' said selected Wavelength divided by pi.

7. A selective sound-absorbing construction for absorbing a greater proportion of sound waves longer than a selected wave-length than of sound waves shorter than said selected Wave-length, said construction comprising sound-absorbing material, sound-reiiecting material acoustically masking a part only of said sound-absorbing material, said sound-reflecting material having apertures therein spaced from each other by a distance approximating one-half of said selected wave-length, and sound-permeable material visually masking said sound-reflecting material and said apertures.

8. A selective sound-absorbing construction for absorbing a greater proportion of sound waves longer than a chosen wave-length than of sound waves shorter than said chosen wave-length, said construction comprising sound-absorbing material positioned to have sound waves impinge upon discontinuous portions thereof, sound-reflecting material acoustically masking other portions of said sound-absorbing material, the area off the sound-reflecting material associated with each of the portions on which sound waves impinge being of a magnitude approximating the square of said chosen wave-length divided by pi, and a layer of sound-permeable material visually masking said sound-absorbing and sound-reflecting materials.

9. An acoustic structure comprising vsoundabsorbing material, a layer of acoustic plaster overlying said sound-absorbing material, and apertured sound-reflecting material on one side of said acoustic plaster for intercepting portions of sound waves traveling toward said soundabsorbing material while allowing other portions thereof to pass through the apertures.

l0. An acoustic structure comprising soundabsorbing material, a layer of acoustic plaster overlying said sound-absorbing material, and apertured sound-intercepting material on one side of said acoustic plaster for masking said sound-absorbingmaterial from portions of sound waves traveling toward it while permitting other portions of said sound waves to pass through the apertures.

l1. An acoustic structure for absorbing a greater proportion of sound Waves longer than a chosen Wave-length than of sound waves shorter than said chosen wave-length, comprising a sound-reflecting layer having apertures therein spaced from each other at a distance approximately one-half of said chosen wave-length, and a layer of acoustic plaster associated with said sound-reflecting layer on one side thereof.

12. An acoustic structure comprising a substantially continuous layer of porous acoustic plaster, and a discontinuous sound-reecting layer adjacent said plaster on the outer side thereof to intercept sound waves traveling towards said layer of acoustic plaster,

13. A selective sound-absorbing construction for absorbing a greater proportion of sound Waves longer than a selected wave-length than of sound waves shorter than said selected wave-length, said construction comprising a substantially continuous layer of porous acoustic plaster, and a sound-reflecting layer adjacent said plaster on one side thereof, said sound-reecting layer having apertures so spaced that the area of said reflecting layer associated with each aperture has a magnitude approximating the square of said selected Wave-length divided by pi.

14. An acoustic structure comprising blocks of substantial size of porous material having beveled edges exposed to sound Waves, and a coating layer acoustically masking the surfaces of said blocks between said beveled edges.

l5. A selective sound-absorbing construction for absorbing a greater proportion of sound Waves longer than any chosen wave-length in the range from one foot to eleven feet than of sound waves shorter than said chosen wave-length, said construction comprising means forming a Wall having areas of relatively high sound-absorbing capacity with spaces between them approximately equal to half of said chosen Wavelength.

VESPER A. SCHLENKER. 

